This invention generally relates to a beverage container. The invention more particularly relates to a hollow, blow molded bottle shaped container of biaxially oriented polyethylene terephthalate. The container is intended to receive a product, such as a liquid, while the product is at an elevated temperature. The container is one of that variety generally known as heat set containers.
Blow molded, biaxially oriented polyethylene terephthalate (PET) containers are known to have a broad spectrum of uses because of their physical properties and characteristics. As they relate to the packaging of liquid beverages, two common classes of PET containers can be distinguished, heat set and non-heat set. The two are distinguished in that heat set containers are adapted to be filled with a liquid while the liquid is at an elevated temperature. This is usually because the liquid has been pasteurized or sterilized. Conversely, non-heat set containers are intended to receive liquids while the liquid is generally at the ambient temperature.
In forming PET containers, an extruded parison or injection molded preform (hereinafter preform for brevity) is initially formed from the PET material. The preform has a substantially cylindrical body which is closed at one end and formed with a mouth opening at the other end which is defined by a neck finish. During the blow molding process, the preform is positioned within a mold cavity where it is held by its neck finish between the mold halves. To impart the biaxial orientation into the PET material, the preform is longitudinally stretched, usually by a push rod, and laterally expanded by the injection of blowing medium into the interior of the preform. As a result of the axial and radial expansion, the side walls of the container, as well as transition areas between the side walls and both the neck finish and base, are biaxially oriented. The biaxial orientation of the container increases the container's stiffness and strength as well as improves its gas barrier properties and transparency. The majority of the neck finish and base, however, remains as substantially amorphous PET.
In the past, most PET containers were used to contain liquids that were initially dispensed into the container at room or a chilled temperature. Ever increasingly, however, there has been more interest in using PET containers for so called "hot fill" applications. In a hot fill application, the product is disposed into the container at an elevated temperature and the container is then is immediately sealed.
Hot fill applications impose additional mechanical stresses on the various container structures. Immediately after the hot fill product is disposed into the container, the temperature of the product causes the rigidity of the PET material to decrease, making the container susceptible to residual and unrelaxed retractive stresses. Additionally, the container must sustain internal pressure changes while maintaining its desired configuration. For example, as the hot fill product cools, it shrinks in volume. This has the effect of inducing a negative pressure within the container which must be withstood both in terms of aesthetic considerations and mechanical consideration.
Even after biaxial orientation, the container lacks thermal stability. Retractive stresses in the oriented side walls result in the PET material having a memory that will contract if subjected to filling with a hot liquid. Residual stresses in the amorphous PET material of the neck finish and base make them susceptible to thermal deformation. To enable the container to withstand the various effects of a hot fill application, the container is heat treated.
During heat treatment, the container is constrained in its molded shape and exposed to an elevated temperature for a predetermined period of time. The heat setting temperature is typically considerably higher than the normal temperature to which the container will be exposed during hot filling and use. During the heat filling process, the PET material rapidly undergoes crystallite melting, recrystallization, growth and structural rearrangements which increase the crystallinity of the PET. PET having an increased crystallinity exhibits a more stable morphology which has a reduced tendency to shrink when exposed to temperatures below that used in heat setting. An initially formed PET container is comprised of both oriented and amorphous regions of PET material. These regions react differently to the heat setting process with the oriented side walls remaining clear after heat setting and the amorphous neck finish and base, as well as the slightly oriented transition areas between these regions and the side walls, becoming opaque. This opacity is a result of the formation of spherite crystallites in the amorphous regions during heat setting. The amorphous regions also exhibit residual stresses, which during the heat setting process, can cause an unsatisfactory degree of shrinkage.
The heat setting and crystallization process relaxes the retractive stresses and reduces the residual stresses in the PET resulting in a more thermally stable container, one with a much reduced tendency to shrink or distort when exposed to fill temperatures below that used in heat setting process. During crystallization, the residual stresses in the amorphous regions tend to cause shrinkage in those areas. Since shrinkage is greatest in those areas with the greatest concentrated stress, excessive amounts of shrinkage can produce an inadequate seal between the closure cap and the neck finish resulting in leakage from the container.
With the above limitations in mind, it is an object of the present invention to provide a PET container suitable for use in hot fill applications.
Another object of the present invention is to provide a PET container which is less susceptible to thermal deformation during heat treating.
Still another object of the present invention is to provide a PET container which exhibits diminished shrinkage as a result of residual stresses in the neck finish.
Yet another object of this invention is to provide a PET container in which a reduced amount of PET material is used in forming the neck finish.
A further object of this invention is to produce a PET container having segmented threads on its neck finish.
Another object of this invention is to provide a container in which the residual stresses are uniformly reduced.
In accordance with the present invention, a blow molded, biaxially oriented PET container, for hot fill application, is formed from a preform whose neck finish has been modified to reduce the thermal deformation effects of the residual stresses in the amorphous PET material. By forming the preform according to the principles of this invention, the neck finish of the blown container is capable of being heat treated and hot filled without experiencing undue shrinkage or other deformation. To reduce the shrinkage which occurs as a result of the residual stresses, the amount of residual stress in the neck finish is itself reduced. This is accomplished by a reduction in the amount of amorphous PET used in forming the neck finish.
As further discussed below, this reduction is residual stress is principally achieved by altering the formation of the threads on the exterior surface of the neck finish. Rather than the typical continuous thread which extends around the neck finish, the present invention substitutes a series of thread segments which are circumferentially spaced apart around the neck finish. The thread segments are generally in the form of a raised ridge which would, if not separated by a gap or recess, extend in a continuous spiral around the neck finish up toward the mouth of the container. Thus, each thread segment extends with the other thread segments along a generally spiraled path. The gaps between the thread segments are circumferentially spaced around the neck finish so that one gap is positioned at approximately 30.degree. intervals. The substantially equidistant circumferential spacing of the gaps helps to assure that the reduction in residual stress is uniformed around the neck finish. As a result of the reduction in residual stress, the neck finish of a container produced according to the principles of the present invention will be less susceptible to thermal distortion or deformation during hot filling and capping thereby resulting in a container that is less likely to leak or exhibit other undesirable attributes which would render the container unusable.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.